An electrodeless discharge lamp lighting device is configured to generate high frequency power with a power amplifier to supply the high frequency power to an electrodeless discharge lamp through an induction coil, while there are different problems generated by fluctuation of a phase difference between voltage and (electric) current of the high frequency power. On account of this, various devices are suggested in which high frequency power supplied to an induction coil is preferably controlled.
For example, a prior art device described in Japanese Laid-open Patent Publication No. H6-76971 comprises an induction coil, a half bridge inverter constituting a power amplifier, a condenser divider for detecting high frequency square-wave voltage from an output point of the inverter to provide a detection voltage, and a current transformer for detecting a high frequency square-wave current from the above output point to provide a detection current. In this device, when a phase difference between the detection voltage and the detection current is reduced, a DC voltage applied to the inverter is lowered, whereas when the phase difference is expanded, the DC voltage is raised. When the phase difference is reduced, load impedance considerably falls, and therefore over-input to the inverter can be prevented by lowering the DC voltage.
A prior art device described in Japanese Laid-open Patent Publication No. H8-45684 comprises an induction coil, a half bridge inverter constituting a power amplifier, a transformer for detecting voltage into each control terminal of the inverter to provide a detection voltage, and a current transformer for detecting high frequency square-wave current from an output point of the inverter to provide a detection current. In this device, when the detection voltage has a delayed phase as compared with the detection current, ON width of the square-wave is reduced while an operating frequency of the power amplifier is decreased. When the detection voltage has an advanced phase as compared with the detection current, ON width of the square-wave is expanded and also the operating frequency of the power amplifier is increased. According to this device, since a phase difference between voltage and current of the high frequency power is reduced, reactive current can be decreased.
When an electrodeless discharge lamp is started, start current flowing through an induction coil must be made larger as its operating frequency is lower. In order to start the electrodeless discharge lamp at every frequency within the operating frequency range, a target value of the start current is set to a current value obtained by adding a margin to the current required at the minimum frequency of the range. This case has a problem that the margin with respect to the maximum frequency of the range becomes too large.
This problem can be solved by a device described in Japanese Laid-open Patent Publication No. 2003-332090. The device comprises an induction coil, a power amplifier, a current transformer for detecting high frequency current from an output of the amplifier to provide a detection current and a F-V converter for detecting the power amplifier's drive frequency corresponding to an operating frequency, and also comprises a set point varying means and a control means. The set point varying means sets a target value of start current to a smaller value as a frequency detected with the F-V converter is higher. The control means controls the drive frequency so that the detection current comes to equal the value set by the set point varying means. According to the device, it is possible to reduce margins of the maximum frequency side in the above range.